Differential output for barometric instrument

ABSTRACT

A BAROMETRIC INSTRUMENT (E.G. ALTIMETER) HAVING TWO BANKS OF ANEROID CAPSULES EACH DRIVING A SEGMENT GEAR EACH OF WHICH IN TURN DRIVES ONE INPUT GEAR OF A DIFFERENTIAL UNIT WHOSE PLANETRY OUTPUT DRIVES THE INDICATING MECHANISM OF THE INSTRUMENT, THE DIFFENTIAL UNIT COMBINING THE MECHANICAL OUTPUTS OF THE ANEROID CAPSULES AND ABSORBIN WITHOUT UNDUE STRESS ANY DIFFERENCES BETWEEN THE OUTPUTS OF THE TWO BANKS.

Feb. 23, 1971 G. A. IRELAND 3,564,922

DIFFERENTIAL OUTPUT FOR BAROMETRIC INSTRUMENT Filed July '7, 1969 2 Sheets-Sheet 1 ALTITUDE COMPU TER INPUT INVENTOR GRAHAM A. IRELAND DOUGLAS L. McNAUGHTON Feb. 23, 1971 A, IRELAND T 3,564,922

DIFFERENTIAL OUTPUT FOR BAROMETRIC INSTRUMENT Filed July 7 1969 2 Sheets-Sheet 2 INVENTORS GRAHAM A. IRELAND DOUGLAS L. McNAUGTON ATTORNEYS United States Patent 3,564,922 DIFFERENTIAL OUTPUT FOR BAROMETRIC INSTRUMENT Graham A. Ireland, Ottawa, Ontario, and Douglas L. McNaughton, Almonte, Ontario, Canada, assignors to Leigh Instruments Limited Filed July 7, 1969, Ser. No. 839,727 Int. Cl. G011 7/12 US. Cl. 73-386 6 Claims ABSTRACT OF THE DISCLOSURE A barometric instrument (e.g. altimeter) having two banks of aneroid capsules each driving a segment gear each of which in turn drives one input gear of a differential unit whose planetary output drives the indicating mechanism of the instrument, the differential unit combining the mechanical outputs of the aneroid capsules and absorbing without undue stress any differences between the outputs of the two banks.

BACKGROUND OF THE INVENTION The present invention relates to a barometric device such as an altimeter having independently-operating aneroid capsules, incorporating differential means for combining the outputs of the aneroid capsules.

Early aneroid assemblies utilized a single aneroid capsule driving a suitable output mechanism such as a connecting arm, crank, and quadrant gear, which in turn drove a pinion attached to a pointer or other indicator. Later assemblies attempted to increase the available output force from the aneroid capsules, by mechanically serially connecting capsules in a bank, or by providing capsules with independent outputs which were combined to provide an increased torque available for driving the indicating mechanism. The use of two independent banks of aneroid capsules each bank of which may include only one aneroid capsule or two or more serially-connected capsules, is desirable because if there is an error in operation of one bank of capsules, it will be reduced by half in the final output if the other band of capsules is operating correctly.

However, if each independent bank of aneroid capsules drives its own output quadrant gear, and each of these quadrant gears directly meshes with a pinion on a common output shaft, the result is a severe mechanical stress applied to the entire assembly if the expansion and contraction of each bank of capsules is not uniform. This problem has been generally solved prior to the present invention, by carefully selecting the capsules in each bank, individually connecting them to their output mechanical linkages, and separately calibrating each half, thereby matching opposing banks of capsules so that the rate of expansion of one bank is substantially equal to the rate of expansion of the other bank. This approach to the problem is time-consuming because of the selection required. Immediate calibration and matching cannot be accomplished with both capsule banks mounted in the altimeter, because of the risk of undue mechanical stresses in the assembly-instead, matching must be tentatively established prior to mounting; and only then can the complete assembly including both mounted capsule banks be calibrated.

SUMMARY OF THE INVENTION According to the present invention, a barometric instrument such as an altimeter is provided in which the use of independent banks of aneroid capsules, each bank driving its own independent segment gear (e.g. a quadrant gear), may be maintained. However, instead of driving the ice output shaft directly, each of the segment gears drives a separate input of a differential gear assembly. The output of the differential assembly is then applied to the indicating means for the instrument.

The use of a differential assembly provides an automatic combining of the mechanical outputs of the segment gears, and eliminates the need for matching of the aneroid capsules prior to their installation in the instrument, enabling the calibration of the instrument without concern about undesirable stresses caused by mismatching, because the differential assembly absorbs differences in rotary displacements without creating undue stresses.

Because of the desirability of minimizing the axial dimensions in the design of barometric instruments such as altimeters, conventional bevel gear differential units may be found to be unsatisfactory in the practice of the present invention, because bevel gear differential units provide at most about a three and one-half to one gear ratio, which necessitates a relatively high axial dimension of the differential assembly. According to a preferred embodiment of the present invention, the differential unit employs face gears, each driven by one of two aneroid inputs, and each driving a common planetary gear through a pair of conventional spur pinions rotatably mounted thereon, the face gears meshing with the pinions. The use of face gears permits gear ratios higher than three and one-half to one, permitting a reduction in the axial dimension as compared with that of a bevel-gear differential assembly.

In a barometric instrument of the type described, means, usually spring-loaded means, must be provided to eliminate backlash. By using a spring-loaded gear in engagement with a differential unit in accordance with a further feature of the invention, the torque provided by the spring is divided by the differential unit and applied to both aneroid inputs, thus making possible the use of a single backlash-eliminating device for the aneroid arrangement.

SUMMARY OF THE DRAWINGS FIG. 1 is a schematic diagram showing an altimeter incorporating a differential unit, in accordance with the principles of the present invention;

FIG. 2 is a sectional view of a differential unit for incorporation into the altimeter illustrated in FIG. 1;

FIG. 3 is an exploded View of the differential assembly of FIG. 2.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS Referring to FIG. 1, aneroid capsules 10, 12, on the left hand side and 14, 16 on the right hand side, are mounted on a central mounting piece 18 which is fixed to the frame of the altimeter (not shown in FIG. 1). A pressure decrease results in expansion of the hub portions of the aneroid capsules and in the case of the left hand pair of capsules 10, 12, is transmitted via a hub connecting post 20 (shown as lying outside the hub assembly, but which may in practice be in the interior of the hub assembly) which is pivotally attached to a link 22 which in turn is pivotally attached to a crank 24 fixed to a rotatably mounted shaft 26. Corresponding elements 28, 30, 32 and '34 are shown connected to the right hand pair of aneriod capsules 14, 16. Because altitude is not linearly related to pressure variation, the linkage 22, 24 and 28, 32 is nonlinear in accordance with conventional altimeter design.

The inward or outward motion of the post 20, caused by contraction or expansion of the aneroid capsules 10, 12, is converted by link 22 and crank 24 to rotary motion of the shaft 26. The shaft 26 is rigidly connected to a quadrant gear 36 of conventional design, and similarly the right hand shaft 34 is connected to a quadrant gear 38. Obviously it is possible to use a segment gear which may have a curved toothed arc not equal to 90; it is conventional in altimeters, however, to use a segment gear that is approximately 90 and therefore can strictly speaking be termed a quadrant gear. The quadrant gears 36, 38, drive pinions 40 and 42 respectively, each of which constitutes an input to a differential gear assembly 44, whose outer periphery meshes with spring-loaded gear 43. A preferred embodiment of the differential assembly is described below with reference to FIGS. 2 and 3. The output of the differential gear assembly is transmitted to an output shaft 46 and gear 49, which in turn drives a spring-loaded coupling generally designated as 48 and which is preferably of the type described in a copending application entitled Flexible Rotary Coupling (Graham A. Ireland et a1.) filed on the same day as the present application. The spring-loaded coupling 48 in turn drives shaft '51 through the rotor 55 of synchro control transformer 53 to shaft 52 and thence to rotary gears 54, 56 and 58 to an output indicator shaft 60 to which is attached an indicating pointer 62. The shaft 60 through bevel gears 70, 72 drive shaft '68 and Veeder-type counter 74. Pointer 62 and counter 74- indicate the altitude reading. For the purposes of the schematic drawing, pointer 62 and mechanical linkage elements 20, 22, 24, 28, 30 and 32 are shown in plan and perspective although the other elements are shown in elevation view.

In accordance with modern practice, altitude readings may also be obtained from altitude computing apparatus (not shown) whose output is applied via control transformer 53 and operational amplifier 64 to a servo motor 66 which also drives shaft 68. Thus, the altitude reading may come either from the servo input via motor 66 and shaft 68 or from the aneroid input via shaft 52. In order that the mechanism function satisfactorily in case the two inputs are not identical, as will often happen, the flexible coupling 48 absorbs the discrepancy in input without unduly stressing any portion of the mechanical system.

The outer casing 57 of the synchro-control transformer 53 is rotatable and may be turned by annular gear 76 fixed to the outer periphery of the synchro-control transformer '53.

Annular gear 76 is driven via idler 78 by pinion 81 mounted on shaft 83. Also mounted on shaft 83 is bevel gear 85 and pinion 91. The bevel gear 85 drives meshing bevel gear 87 which in turn drives barometric counter assembly 89, which is preferably of the type described in a copending application entitled Non-Linear Counter filed on the same day as the present application in the name of J. R. B. Steacie. Shaft 83 may be turned manually by a knob 97 attached to one end of the shaft 83.

The entire casing for the aneroid portion of the unit terminates in an uppermost plate 93 whose outer periphery is toothed to mesh with pinion 91. Bearings 95 interposed between shaft 46 and the plate 93 permit the entire barometric assembly to rotate about the shaft 46 as the knob 97 is turned. Turning the knob 97 also has the effect of rotating the annular gear 76 and thus the outer casing of the synchro-control transformer 53.

The purpose of having the manually adjustable knob 97 and the mechanism associated immediately therewith is to permit the operator of the instrument to set the counter 89 of the indicating mechanism to a specified datum pressure, in dependence upon the prevailing barometric pressure. According to the conventional specifications, the altitude computer will be operating at an assumed barometric pressure of 29.92 inches of mercury. The operator of the altitmeter, an aircraft pilot, will have to be able to adjust the instrument to some other datum barometric pressure for the purpose of landing and taking off, because it is essential that the instrument give correct altitude reading according to actual prevailing conditions at the critical landing and ta'ke-off times. The details of the barometric setting mechanism are not a part of the present invention.

Referring now to FIGS. 2 and 3, the differential assem- 4 bly generally indicated by 44 in FIG. 1, is shown in more detail. The quadrant gear 36 meshes with and drives toothed hub 100 of the first differential input face gear 102 which has teeth 104 on its inner face. The hub 102 is freely rotatable on output shaft 106.

Similarly, the quadrant gear 38 meshes with and drives opposing input face gear 108 by its toothed hub 1|10, also freely rotatable on the output shaft 106.

Positioned between the input face gears 102 and 108 is a planetary assembly generally designated as 112, comprising a pinion support wheel 1114 fixed to the output shaft 106-, and which may be provided with lightening holes 118. Mounted in the support wheel 1.14 are diametrically opposed shafts 124, 126, on which are radially rotatably mounted (i.e., mounted so that their axis of rotation is radial with respect to support Wheel 114) pinion gears 128, 130, each of which meshes with both of the face gears 102, 108. The outer periphery 132 of wheel 1r14 is provided with teeth which mesh with a spring-loaded gear 43 (-FIG. I) loaded by coil spring 122 and mounted in the frame of the altimeter to eliminate backlash in the pivots, links, arms and gears of both sides of the aneroid assembly. The backlash spring force is thus divided and balanced by the differential unit so that both capsule banks are equally loaded at all times, and friction in both of the drive systems feeding the differential unit is minimized.

Holding the differential assembly together are clip rings 13 4, 136 engaging recesses 138, 140 on the output shaft 106.

The use of face gears with conventional pinion gears in the planetary permits gear ratios in the differential unit higher than the approximately three-and-a-half to one maximum ratio possible using bevel gears. This is advantageous where it is desired to minimize the axial dimensions of the differential unit, which is frequently the case in altimeter design.

In operation, it can be seen that circumferential motion of the quadrant gears 36 and 38 in opposite senses results in rotation of the face gears 102, 108, in the same direction, because of the opposite positioning of the quadrant gears one on either side of the output shaft. However, the two quadrant gears need not rotate through exactly the same angle for a given pressure change causing expansion or contraction of the aneroid capsule banks that drive the quadrant gears. If one of the quadrant gears moves through a greater angle than the other, the result is simply that the planetary system 112 absorbs the difference without stressing the assembly. This is one property of a mechanical differential unit of this type. The other property of the unit is, of course, that the output rotation of the planetary system is the mechanical average of the two inputs, and the output torque the sum of the two inputs. seal The description above relates to a two-bank aneroid assembly driving a differential output. However, it is ob vious that additional banks could drive additional differential assemblies so as to combine the outputs of more than two banks of aneroid capsules. Other departures from the specific embodiment illustrated and described will occur to thoes skilled in the technology; as a trivial example, three pinions spaced apart might be used in the planetary assembly in lieu of the two diametrically opposite pinions 128, 160.

What we claim as our invention is:

1. In or for use with a barometric instrument having at least two pressure-responsive devices each providing a rotary mechanical output varying with ambient pressure, the improvement comprising a differential assembly having a first rotary input gear driven by the output of one of the pressure-responsive devices, a second rotary input gear coaxial with the first input gear and driven by the output of another of the pressure-responsive devices, and a rotary output assembly in planetary relationship with and driven by both said input gears.

2. The improvement defined in claim 1, wherein the rotary input gears are face gears and wherein the rotary ouput assembly comprises a support element coaxial with both said face gears and bearing radially rotatably mounted pinions each meshing with the teeth of both said face gears.

3. A barometric instrument comprising a first aneroid bank including at least one aneroid capsule providing a first linear mechanical output in response to ambient pressure variations, a second aneroid bank including at least one aneroid capsule providing a second linear mechanical output in response to ambient pressure variations, first crank means connected to the first linear mechanical output and converting the first linear mechanical output to a first rotary output, second crank means connected to the second linear mechanical output and converting the second linear mechanical output to a second rotary output, and a differential assembly having a first and a second input gear coaxial with one another and a planetary assembly coaxial with the input gears and having radially rotatably mounted pinions each meshing with and driven by both said input gears whereby the rotation of the planetary assembly is the average of the rotations of the input gears, the first input gear being driven by the first rotary output and the second input gear being driven by the second rotary output.

4. A barometric instrument as defined in claim 3, additionally including indicating means driven by the rotation of the planetary assembly.

5. A barometric instrument as defined in claim 4, wherein each of the input gears is a face gear whose face teeth mesh with said pinions.

6. A barometric instrument as defined in claim 4, additionally including a spring-loaded gear mounted in the instrument for the purpose of eliminating backlash in the system, wherein the outer periphery of the planetary assembly is provided with teeth meshing with the springloaded gear whereby the spring force provided by the spring-loaded gear is applied via the difierential assembly to each said aneroid bank.

References Cited UNITED STATES PATENTS 3,154,944 11/1964 Johanson 73182 DONALD O. WOODI EL, Primary Examiner 

